1. Field of the Invention
The present invention relates generally to an abnormality detecting apparatus for use in a valve timing control apparatus for an internal combustion engine, which controls the valve timing of either the intake valve or the exhaust valve of the internal combustion engine in accordance with the running conditions of the engine. More particularly, this invention relates to an abnormality detecting apparatus that is used for an internal combustion engine with a variable valve timing mechanism and detects an abnormality in a displacement angle of the cam shaft with respect to the crankshaft.
2. Description of the Related Art
Variable valve timing mechanisms have been put into practice, which alter the valve timing of either the intake valve or the exhaust valve in accordance with the running conditions of the engine. One known variable valve timing mechanism of this type continuously varies the rotational phase difference (displacement angle) of the cam shaft with respect to the crankshaft.
Engines having a variable valve timing mechanism perform such feedback control as to converge the real rotational phase difference (real displacement angle) to the target rotational phase difference (target displacement angle) that is determined based on the running conditions of the engines. It is therefore necessary to accurately detect the real displacement angle using a phase difference detecting apparatus as described in Japanese Unexamined Patent Publication No. Sho 59-105911.
This publication discloses an apparatus that accurately detects the real displacement angle by measuring the interval between a first pulse signal output for each rotation of the crankshaft and a second pulse signal output for each rotation of the cam shaft. Engines with such a variable valve timing mechanism have inherent problems originating from the variable valve timing mechanism.
The first problem is that when the variable valve timing mechanism fails due to the entrance of some foreign matter or the like, the real displacement angle cannot be converged to the target displacement angle. If various control amounts such as the fuel injection timing are so determined as to be associated with the target displacement angle, the output characteristic of the engine will be poor. It is therefore necessary to determine whether or not a failure has occurred in the variable valve timing mechanism and correct various control amounts, if such has occurred, to prevent the output characteristic of the engine from deteriorating.
In this respect, typical failure detectors have been proposed that determine that the variable valve timing mechanism has failed when the absolute value of the difference between the target displacement angle and the real displacement angle is greater than a predetermined threshold value. However, an erroneous failure detection is apt to occur if it is determined that the variable valve timing mechanism has failed when the absolute value of the difference between the target displacement angle and the real displacement angle is greater than a predetermined threshold value. This is because the absolute value of the difference between the target displacement angle and the real displacement angle is generally large at the initial stage of the control of changing the real displacement angle toward the determined target displacement angle. If the threshold value for the determination of a failure is set small, it is frequently determined that the variable valve timing mechanism has failed. If this threshold value is set large, on the other hand, it is not possible to execute the effective failure determination.
If the target displacement angle is the maximum advance angle or lies near the maximum delay angle, the real displacement angle may stop changing even when it is smaller or greater than the target displacement angle. In other words, because the allowable operational ranges of variable valve timing mechanisms differ from one mechanism to another due to the allowances for constituting components, the real displacement angle may physically become the maximum shifted state (where the constituting components abut on one another) before reaching the maximum or target displacement angle, so that the real displacement angle does not converge to the target displacement angle. When the threshold value used for failure determination is constant, therefore, the occurrence of a failure may be erroneously determined even if the real displacement angle is changed to the maximum level from the viewpoint of the variable valve timing mechanism.
If the determination of the proper recovery from a failure in the variable valve timing mechanism is executed when the absolute value of the difference between the target displacement angle and the real displacement angle becomes smaller than the threshold value for failure determination, the proper recovery may be erroneously determined. As the target displacement angle varies with time in accordance with the running conditions of the engine, the absolute value of the difference between the target displacement angle and the real displacement angle also varies.
The absolute value of the computed difference merely indicates the transient operational condition of the variable valve timing mechanism, not the steady operational condition thereof. Accordingly, the variable valve timing mechanism may be determined to have recovered properly in some cases even if it is still in a failure state from the steady viewpoint.
If failure determination and the determination of the proper recovery are executed in a normal decision time when the response characteristic of the variable valve timing mechanism is poor, those determinations may result in error. With regard to a variable valve timing mechanism that is driven hydraulically, for example, the operational resistance is high when the oil viscosity is low. A longer time is needed in this case for the real displacement angle to converge to the target displacement angle than in the case of the normal oil viscosity. If the time for failure determination is set based on the normal oil viscosity, therefore, the variable valve timing mechanism may be determined to be in a failure state even though the real displacement angle is varying toward the target displacement angle.
The second problem is a variation in the engine characteristics, which is caused by the shifting of the displacement angle of the cam shaft with respect to the crankshaft due to the improper setting of the timing belt or the like. More specifically, even when the valve timing is changed by altering the displacement angle by the variable valve timing mechanism, a predetermined valve timing cannot be established and the desired engine characteristics cannot be acquired.
Generally, various control timings for engines are determined on the basis of the rotational angle of the crankshaft (crank angle), so are the opening/closing timings of the intake valve and exhaust valve. Many intake valves and exhaust valves are so driven by the cam shaft as to accomplish predetermined opening and closing timings. This cam shaft is driven by the crankshaft with a transmission member such as the timing belt or timing chain.
If the displacement angle of the cam shaft with respect to the crankshaft shifts from the predetermined displacement angle because of the improper teeth engagement of the timing belt, the improper setting of the timing belt or the like, the intake valve and exhaust valve cannot be opened or closed at the predetermined opening/closing timings, thus deteriorating the engine output characteristics.
In view of the above situation, a timing failure detecting apparatus has been proposed that normally determines if the displacement of the angle (timing) of the cam shaft (the displacement angle) with respect to the crankshaft lies within a predetermined range based on the crank angle detected by the crank angle sensor and the cam angle detected by the cam angle sensor. This detecting apparatus always detects the displacement angle of the cam shaft with respect to the crankshaft and detects a timing failure when the displacement angle comes off the predetermined range.
The shifting of the displacement angle of the cam shaft with respect to the crankshaft, which is caused by the improper setting of the timing belt or the like, is not inherent merely to engines having a variable valve timing mechanism. However, engines having a variable valve timing mechanism achieve the optimal valve timing that varies according to the engine condition, by changing the displacement angle of the cam shaft with respect to the crankshaft.
While the cam shaft is changing toward the target displacement angle, the displacement angle of the cam shaft with respect to the crankshaft is continuously changing and never stays at a constant value. Consequently, if a failure is determined by checking whether the displacement angle lies within a given threshold value as is done for ordinary engines, there is a possibility of erroneous detection. Even in the case where a failure is detected after the cam shaft reaches the target displacement angle, an abnormal displacement angle cannot be detected using a threshold value of "1" because the cam shaft can take various target displacement angles.
One may think that a predetermined value as the threshold value for failure determination should be changed for each target displacement angle so that the timing failure would be determined based on such a variable threshold value. In changing the cam shaft toward the target displacement angle, the real displacement angle may not match with the target displacement angle because of the response delay of the cam shaft. If it is determined whether or not the displacement angle lies below a predetermined threshold value for failure determination while changing this predetermined value simply for each target displacement angle, it is likely that a failure is erroneously detected even though no failure has actually occurred in the displacement angle.